The invention relates to furnaces such as kilns used for firing ceramic wares, brick work, and the like.
In the final stages of fabrication, ceramic wares are heated or "fired" in a kiln to achieve their final hardened condition and to cure the glazes applied to the wares' surfaces. Many complex changes take place during the firing stage in the ceramic material itself and in the glaze. Water and absorbed gases are driven out of the ceramic material, microscopic structural rearrangements occur, and chemical reactions occur in the glazes. If the firing is not performed properly, the wares can crack or even explode, and the glazes can run or yield false, unexpected colorations.
The changes which take place during firing are highly sensitive to temperature. Numerous firing schedules, according to which the wares are fired for prescribed periods in prescribed temperature ranges, have been specially devised to facilitate the changes. For reliable and reproducible results the wares must be subjected to uniform, accurately controlled temperatures.
Typical kiln operation has three stages: a preheat stage, a soak stage, and a cooling stage. In the initial, preheat stage the kiln temperature is gradually increased from ambient room temperature to the soak temperature, at which the wares cure. During the preheat stage the elements providing the heat source for the kiln are fired at only a fraction of their maximum capacity. In the soak stage the kiln heating chamber is maintained at a constant high temperature or, according to some firing schedules, is advanced over several distinct high-temperature levels. During the soak stage the wares undergo the various chemical and physical processes producing the final hardened condition of the wares and/or glazes. In the final cooling stage the wares are gradually cooled to a temperature which will allow the kiln to be opened and unloaded.
For proper firing the wares must be heated gradually and uniformly. Local regions of overheating or underheating can produce wares having weak spots or discoloration. Excessive overheating can cause the wares to crack or explode. Kilns are typically heated by introducing heated gas either from a burner or electrical heating unit. The regions of the kiln heating chamber in the vicinity of the ports at which heated gas is introduced into the chamber naturally tend to be hotter than other regions of the chamber. These regions are inherent hot spots in any kiln construction. Wares positioned close to the gas-entrance ports tend to cure differently from wares stacked in the inner portions of the heating chamber. Several methods have been attempted in the past to counteract this natural tendency and to provide a more uniform and controllable temperature distribution throughout the heating chamber.
In one primitive approach, sacrificial wares are placed on the shelves or racks in the vicinity of the heat source so as to shield the other wares. It is expected that the sacrificial wares will be improperly fired and may even crack, but they nevertheless serve to protect the remaining contents of the heating chamber. This method is wasteful and uncontrollable. The full shelf space of the kiln is not utilized for producing finished wares, and cracking of the sacrificial wares is unpredictable. Whether or when a sacrificial ware may crack and fail to provide adequate shielding is beyond control.
In another approach a baffle arrangement of one kind or another shields the wares to be fired from the heating source. A common baffle arrangement is the bag wall. A typical bag wall is constructed of fire brick stacked sideways to form a wall somewhat removed from the burners providing the heat source and extending from the kiln floor to the level of the burners. The bag wall shields the wares within the heating chamber from direct contact with the flame or flow of heated gas from the burners. The bag wall may also be used in an attempt to control the uniformity of the temperature distribution within the usable region of the heating chamber. Selected bricks may be removed from the bag wall to open up holes through which the heated gas may flow. Control over the circulation of the heated gas in this manner provides some measure of control over the uniformity of the temperature distribution.
In a typical operation the wares are loaded onto shelves or racks within the kiln heating chamber and subjected to heat for an extended period of time, sometimes as long as several days. During this time the kiln remains sealed until the firing is complete, so that further wares cannot be added during the firing process. Thus, shelf space, empty at the time the kiln is fired, remains empty until the next firing. For a commercial kiln unused shelf spce, or heating chamber volume dedicated to a bag wall or the like, represents a loss of potential profitability. It also represents a loss of fuel economy since roughly the same quantity of fuel must be expended throughout the firing process whether the racks for the wares fully or partially occupy the heating chamber volume and whether the racks are fully or partially loaded with wares. Known kiln constructions and arrangements for controlling the heat flow either do not achieve a high degree of temperature uniformity or do so only at the sacrifice of valuable heating chamber volume.